1. Field of the Invention
The present invention relates to an electric parking brake apparatus capable of operating a parking brake of a vehicle by use of electric drive means such as an electric motor.
2. Description of Related Art
Japanese Patent Application Laid-Open (kokai) No. 2002-225701 discloses a conventional electric parking brake apparatus of such a type. The disclosed apparatus includes a parking brake, which consists of a brake disk and a brake pad for generating a braking force corresponding to a pressure-contact force at the time of pressure contact with the brake disk; and an electric motor for driving the brake pad. Parking brake activation control is performed in such a manner that the motor is operated until current flowing through the motor and increasing with the pressure contact force reaches a predetermined target value, so as to bring the parking brake into an activated state in which the parking brake can stop the vehicle and maintain it in the stopped state, and subsequently, the operation of the motor is stopped.
Further, in the disclosed apparatus, between the motor and the parking brake is disposed a speed reduction mechanism including a plurality of gears and adapted to permit transmission of drive torque of the motor to the brake pad, while preventing transmission to the motor of a force stemming from the pressure contact force of the brake pad. Therefore, even after the operation of the motor is stopped after completion of the parking brake activation control (even after the current flowing through the motor decreases to zero), the pressure contact force can be held, whereby the vehicle can be stopped and maintained in the stopped state.
Incidentally, a parking brake as described above is typically disposed in the vicinity of a service brake of a vehicle, and therefore, heat generated at the service brake is transferred to the parking brake. Accordingly, when the temperature of the service brake becomes high as a result of repeated use of the service brake during traveling of the vehicle, the temperatures of components (brake disk, brake pads, etc.) of the parking brake also become high, with resultant thermal expansions of the components. Therefore, when the parking brake is brought into an activated state in such a condition by means of the above-described parking brake activation control and the braking force of the parking brake is held by the above-described action of the speed reduction mechanism, depending on the relation between thermal expansion coefficients of the components of the parking brake, the braking force of the parking brake may decrease as the temperatures of the components decrease with time, because of differences in thermal contraction/expansion among the components.
In order to solve such a drawback, the disclosed apparatus is configured in such a manner that even after the parking brake enters an activated state upon completion of the above-described parking brake activation control, the parking brake activation control is repeated every time a predetermined time elapses, to thereby maintain the braking force of the parking brake at a proper level.
However, the disclosed apparatus has the following problem. The temperature decrease speed of each component of the parking brake depends on the temperature difference between the temperature of the component and the ambient temperature, and increases with the temperature difference. Therefore, when the temperatures of the components of the parking brake are extremely high at the time of completion of the parking brake activation control, the temperature decrease speeds of the components after that time become high, so that the braking force of the parking brake decreases considerably before the above-described predetermined time elapses after that time. As a result, the disclosed apparatus may fail to maintain the braking force of the parking brake at a desired level through the elapse of the predetermined time.
Moreover, in the case where the braking force of the parking brake drops because of a factor other than the temperature decreases of the components of the parking brake at a certain point in time after completion of the above-described parking brake activation control, as in the above-described case, the disclosed apparatus may fail to maintain the braking force of the parking brake at a desired level through the elapse of the predetermined time.
Meanwhile, it has been known that even when the pressure contact force of the brake pad against the brake disk is constant, the braking force of the parking brake decreases when the temperatures of the components of the parking brake increase, because of a drop in friction coefficient of the friction surface of the brake pad. However, in the disclosed apparatus, a target value of current flowing through the motor (a target value of pressure contact force) used in the above-described parking brake activation control is a constant value irrespective of the temperatures of the components of the parking brake. Therefore, in the case where the temperatures of the components of the parking brake at the time of completion of the parking brake activation control are extremely high, the parking brake may fail to generate a necessary braking force even at that time. As described above, the disclosed apparatus has a problem in that after the parking brake enters an activated state upon completion of the parking brake activation control, the parking brake may fail to properly maintain a necessary braking force, depending on various stop states of the vehicle.